If an onen comes across a meis mitzvah (let's assume there is either another person to be involved with the onen's personal meis or there is more time to be involved with his meis later), is the onen allowed to be involved with the meis mitzvah? Is it optional for the onen? Obligatory? Perhaps ossur? What's the proof for this?
The first Rashi on Daf 20a in ברכות teaches us the underlying principal of Meis Mitzva:
A Meis Mitzva takes precedence over other Torah-prescribed activities - including prohibitions and positive Mitzvoth - because of Kovod HaBrioth, dignity to humans.
The Gemara there - and in other places (like Nazir 47a) - analyses why certain people have to bury the Meis Mitzva, including:
Similarly we see in Daf 3b of מגילה that Meis Mitzvah takes precedence over prohibitions:
There the Gemara defines precedences:
That said, the Gemara on 7a in יבמות mentions that Shabbat observance has precedence over Meis Mitzvah.
Even if we assume that the Torah forbids an Onen from doing Mitzvoth, nevertheless Kovod HaBrioth would override that - as we see that it overrides positive and negative commandments.
He would therefore be obligated to bury the Meis Mitzva, possibly even at the expense of delaying his own relative's burial.